There is a conventional hybrid drive device wherein the rotary power of an engine is divided by a power splitting mechanism, transmitted to an axle and a first motor generator or a second motor generator, performs regeneration by one of the motor generators of the first motor generator and second motor generator, and drives the wheels by the other motor generator.
For example, in patent literature 1, an electro-mechanical transmission is disclosed that has: an input member for receiving power from a prime mover power source; an output member for transmitting output power from the transmission; a first and second motor generator; an energy storage unit for mutually exchanging electric power between the first and second motor generator; a control unit for adjusting the mutual exchange of electric power between the energy storage unit and the first and second motor generators, and adjusting the mutual exchange of electric power between the first and second motor generators; and three planetary gear devices (sets) that are coaxially matched; wherein each planetary gear device uses a first and second gear member; each of which engages with a plurality of planetary gears that are installed in a carrier that is assembled in each corresponding planetary gear device; the first and second motor generators are coaxially aligned with each other and with the three planetary gear devices; at least one of the gear members of the first or section planetary gear device is connected with the first motor generator, and at least one of the gear members of the first or second planetary gear device is connected with the second motor generator; there is a means for functionally connecting the carriers that are connected with the first, second and third planetary gear devices with each other and with the output member; of the gear members of the first or second planetary gear device, one gear member that is not connected with the first motor generator is continuously connected with one of the gear members of the third planetary gear device; of the first or second planetary gear device, the gear member that is not connected with the first motor generator is functionally connected with the input member; and the gear member of the third planetary gear device that is not connected with the first or second planetary gear device is selectively connected to ground.
With the electro-mechanical transmission disclosed in patent literature 1, by controlling the rotational speed (rpm) of the first motor generator it is possible to set the engine rpm to an rpm that optimizes the fuel consumption, and when doing that, by having the first motor generator generate electric power, and then driving the second motor generator with that electric power, it is possible for the overall device to obtain necessary and sufficient torque at the output shaft. Reportedly, the electro-mechanical transmission has two operating modes; a first mode that is set in the relatively low-speed state at the start of movement and immediately afterwards, and a second mode that is set in a state when the speed of the vehicle has increased a certain extent. More specifically, in the first mode where a relatively large driving torque is necessary, by fixing the ring gear of the planetary gear mechanism that is on the output shaft side, this planetary gear mechanism is caused to function as a reduction gear train, however, in the second mode where the speed of the vehicle has increased to a certain extent, the sun gear and carrier of that planetary gear mechanism are connected and integrated so that the planetary gear mechanism does not perform an operation for increasing or decreasing the speed.
Patent literature 2 discloses a hybrid drive device in which an internal combustion engine and a first electric motor and second electric motor having a generating function are connected to an output member via a gear mechanism that includes a planetary gear mechanism, wherein there are a pair of planetary gear mechanisms, and each planetary gear mechanism has three rotating elements that mutually perform differential operation; there are at least three engagement devices that switch the path for transmitting power to the output member via these two planetary gear mechanisms; and the power transmission path is set to three different ways in which the mutual relationship between the operating states of the first electric motor and second electric motor for setting the ratio of the rpm of the internal combustion engine and the rpm of the output member to a specified value differs according to the engaged state and released state of the three engagement devices. In this hybrid drive device, the mutual relationship of the operating state of each electric motor when a specified transmission gear ratio is set differs for each power transmission path. Therefore, when mounted in a vehicle, by selecting a suitable power transmission path for low speed, medium speed and high speed, it is possible to operate the internal combustion engine in an efficient state, and power transmission by way of electric power of each electric motor is suppressed, so as a result, power is transmitted efficiently to the output member at any speed range, and thus it is possible to improve fuel consumption.    [Patent Literature 1] Japanese Kokai Publication No. 2000-62483A    [Patent Literature 2] Japanese Kokai Publication No. 2006-282069A